Age of beef heifers at first mating

The age at which beef heifers should be first mated depends upon the economics of management input against returns. Well grown British breed heifers can be mated as early as 15 months to calve at 24 months, but require extra managerial effort if good results are to be obtained.

The two main advantages of mating heifers as yearlings instead of two- year-olds are:

Heifers mated as yearlings produce an extra calf during their life in the herd.

High producing mothers can be identified earlier, which allows for a shorter generation interval and hence, quicker genetic progress.

The main cost of mating heifers as yearlings is the need to feed weaner heifers so that they achieve a minimum liveweight at mating. British breed heifers should weigh at least 275-280 kg at joining, while the later maturing European breeds should weigh at least 320 kg.

When returns from beef cattle are good, it is worth bringing heifers into production as early as possible. However, there may be less incentive to calve heifers as two-year-olds if seasonal conditions are poor, as extra feed costs may not be offset by the extra return. However, even in poorer seasons, the value of quickened genetic progress must be considered.

Lifetime performance of heifers calving at two years

Well-managed heifers calving first at two years have a higher lifetime productivity than heifers calving first at older ages. Although some producers believe that heifers calving at two years will have retarded growth and a subsequent reduction in productivity, a number of long-term experiments have shown that this is not the case. For example, the results of an experiment using Hereford cattle are summarised in the following table:

Table 1. Summary of lifetime productivity

Age at first calving

Two years

Three years

No. females started in experiment, 1948

59

60

No. remaining in 1962

23

22

Average mature bodyweight in 1956, kg

521

534

Total No. calves weaned

533

482

Average weaning weight of calves (corrected for calf sex) kg

216

220

It can be seen that calving first at two years had no effect on cow survival and life span in the herd, virtually no effect on mature cow size, but a large effect on productivity. The 59 two-year-old calvers produced an extra 51 calves during their lifetime in the herd.

Fertility of heifers mated as yearlings

Low pregnancy rates and poor calving percentages in mobs of heifers mated as yearlings are usually associated with inadequate liveweight at mating, as liveweight is the most important factor determining the onset of oestrus. Pregnancy rates of more than 85% can be regularly achieved with yearling heifers that have reached critical weight before mating.

Calving difficulty

Calving difficulty was commonly believed to result from mating heifers at too young an age. However, calving difficulty is now known to be a problem of first-calf heifers, whether they calve first at two years or three years.

Well-grown mated yearlings should have no more difficulty calving than heifers joined first as two-year-olds, despite the more mature frame of three-year-old heifers. This is because calf size increases with the increased frame of the dam, and older heifers tend be fatter at calving, both of which can lead to calving difficulty.

Growth of heifers mated as yearlings

It is commonly believed that early calving can seriously retard the growth of heifers. However, as Table 1 indicates, calving at two years has practically no effect on the mature bodyweight of cows. More detailed experiments have shown that early calving may delay body development for up to 12 months, but almost the same mature size is reached. This effect can be minimised if the heifers are fed well after calving.

Fertility after first calving

Heifers, particularly those calving at two years, take considerably longer after their first calving to return to heat. In many cases, heifers may become pregnant late or fail to conceive altogether. This can be overcome by ensuring that only well grown heifers are mated, that the heifers calve down in good body condition, (condition score 3), and are well fed after calving to be joined at condition score 2.5.

Joining heifers a month earlier than the main herd is a management option that enables the heifers more time after calving to begin cycling before the start of mating.

Milk production and calf growth

Young cows calving at two years produce less milk and wean lighter calves than older cows, but can still produce good calves if calved in good body condition and are well fed after calving.

Although the calves of yearling joined heifers are lighter at weaning, following calves are as heavy as if the heifers had calved at three years. The first calf of yearling joined heifers, although lighter, therefore represent an extra year of production and genetic gain, as opposed to heifers calving at three years of age.

Geoff Kroker, Bendigo and Lisa Clarke, Hamilton June, 2000Create New

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